1. Field Of The Invention
This invention in general relates to brakes and in particular combined drum and disk brakes in one wheel.
2. Prior Art
In common motor vehicle brake systems, applying a foot brake action causes an hydraulic pressure in a master cylinder. The pressure is transmitted to all wheels that contain either drum or disc brakes.
Front wheel brakes are usually of the disc type with static calipers, while rear wheel brakes are typically of the drum type.
Typical drum brake systems use left and right brake shoes which are lined with friction pads. Both drum and disc brake systems operate by applying hydraulic pressure to pistons which force stationary friction producing members into contact with rotating friction producing members.
In a disc type braking system, when a sufficient amount of pressurized brake fluid is supplied to a wheel brake cylinder, calipers will pinch the rotating disc between the friction pads thus causing a frictional braking action.
Drum brakes are usually of the duo servo type having an hydraulic wheel cylinder with two pistons acting in opposition. Hydraulic brake fluid in said cylinder under pressure from a master cylinder will drive pistons in the wheel cylinder to drive shafts that will drive the ribs of the drum brake shoes radially outward to the peripheral drum surface which will engage the drum brake shoe friction pads and produce frictional vector forces and a desired braking effect.
Biased springs appropriately placed, keep the friction pads away from the friction surfaces when the foot brake is not activated.
By certain laws of physics, the amount of friction force that develops between two surfaces in relative motion and rubbing each other, depends upon the coefficient of friction and the normal(perpendicular) force holding them together. The contact area is not a factor that will proportionately increase or decrease the friction force.
Hence, in either a drum brake system or a disc brake system in separate wheels, increasing the area of the friction pads will not make the vehicle stop faster if the amount of applied brake pressure developed and used when the foot brake pedal is asserted is the same as for larger brake pads; the only advantage gained will be longer lasting brake pads.
Therefore it would be difficult to increase the brake pad efficiency or power efficiency as economically on a drum brake or disc brake system when each is in a separate wheel.
A search for combined brake systems was made but none could be found. Burnett et al U.S. Pat. No. 3,899,052 was examined but it was clearly a disc brake device. Cochran et al U.S. Pat. No. 3,838,755 was also clearly a disk braking mechanism. U.S. Pat. No. 4,450,947 was titled "Combination Friction Ring and mount therefor . . . " but the word "Combination" does not connotate a combining of drum and disk brakes in one wheel.
These were viewed to see if any evidence of combined drum and brake systems was there. Many other patents under the Brakes classification were studied but generally they were distinctly of the disc, drum, or conical variety.
None had the advantages of this invention which will be described clearly subsequently in this application for patent.